Improve error messages when hitting PHP file upload issues

Summary: See T429. When you hit certain errors, you get less-than-helpful
messages like "upload error 3". Instead, produce human-readable errors.
Test Plan: Simulated errors, verified user receives decent error messages.
Reviewed By: aran
Reviewers: jungejason, tuomaspelkonen, aran, startupguy
CC: aran
Differential Revision: 816